1. Field of the Invention
The invention described herein relates to a laminated composite of a ceramic and a refractory metal and, more particularly, to a composite which can be fabricated to form parts for a helical expander for the energy generation industry. The ceramic includes silicon nitride.
2. Description of the Prior Art
Efforts have been made to construct a helical expander to recover heat energy by directly expanding combustion gases in coal-burning power plants. It is expected that, if a suitable expander were available, the typical generating efficiency of these plants could be increased from 34-40% to 50-60%.
These efforts have failed because the materials considered for constructing the expander could not survive the environment for a sufficient period of time to be practical. The gases from the combustion are typically at a temperature on the order of 1000.degree. C. and at a pressure on the order of 240 MPa (megapascals.). Further, the stream includes corrosives, such as hydrogen sulfide and sulfuric acid.
Helical expanders having a ceramic rotor mounted on a metal shaft have been considered. The ceramic would provide protection against heat and corrosives and the metal would provide strength. A suitable match of a ceramic and metal was not been found. In particular, a bond of sufficient strength to secure the ceramic rotor to the metal shaft is a problem. For instance, the coefficient of thermal expansion for the ceramic and for the metal must be quite similar for the bond between the ceramic rotor and metal shaft to survive thermal cycling.
Silicon nitride (Si.sub.3 N.sub.4), a ceramic which has been employed in stator blades of high-temperature gas turbines, has been suggested as the ceramic from which to fabricate parts for the helical expander. Characteristics which were considered in suggesting silicon nitride include thermal shock resistance, oxidation resistance, thermal creep resistance, environment, and fabrication. However, suitable bonding of silicon nitride to a metal, in terms of strength, has been achieved in the past only at temperatures above 1400.degree. C. and pressures above 137 MPa. This bonding appears to depend on partial melting or diffusion of the metal into the silicon nitride which is porous and these temperatures and pressures have adversely affected the characteristics of the silicon nitride and/or metal in most cases.